In a dna double helix an adenine of one strand always pairs with a(n) of the complementary strand, and a guanine of one strand always pairs with a(n) of the complementary strand. in a dna double helix an adenine of one strand always pairs with a(n) of the complementary strand, and a guanine of one strand always pairs with a(n) of the complementary strand. uracil cytosine cytosine thymine guanine adenine thymine cytosine cytosine uracil